A fuel cell is a generator that generally converts chemical energy into electrical energy through oxidation and reduction of hydrogen and oxygen.
Since a unit cell of the fuel cell generates too low a voltage to be used alone in practice, the fuel cell generally has several to several hundred unit cells stacked therein. When stacking the unit cells, separators or bipolar plates are used to facilitate electrical connection between the unit cells and to separate reaction gases in the stack of the unit cells while acting as a fluid passage through which a water coolant flows.
The separator or bipolar plate is an essential component for a fuel cell along with a membrane-electrode assembly (MEA) and has a variety of functions, such as structural support for MEA and gas diffusion layers (GDLs), collection and transmission of current, transmission and removal of reaction gas, transmission of the water coolant used for removing reaction heat, and the like.
Hence, it is necessary for a material for the bipolar plate to have good electrical and thermal conductivity, air-tightness, and corrosion resistance.
When a bipolar plate is made of a metallic material, there are many advantages in that volume and weight reduction of a fuel cell stack can be accomplished via thickness reduction of the bipolar plate, and in that the bipolar plate can be fabricated by stamping and the like, which facilitates mass production of the bipolar plates.
Stainless steel, aluminum alloys, carbon steel plates, and the like are proposed as candidate materials for the bipolar plate for fuel cells.
However, the use of the metallic material for the bipolar plate leads to formation of a passive film on the surface of the bipolar plate, which influences electrical conductivity. Further, considering high temperature and high humidity operating environments of the fuel cell, the electrical conductivity is likely to exhibit a gradual decrease due to a thickness increase of an oxide film on the bipolar plate and the bipolar plate can suffer functional deterioration resulting from corrosion when the fuel cell is used for a long period of time.
To solve such problems, attempts have been made to enhance corrosion resistance and electrical conductivity by increasing the contents of chromium and nickel in the stainless steel for the bipolar plate while removing the oxide film from the surface of the bipolar plate by etching the passive film.
However, the increase in contents of chromium and nickel in the stainless steel causes not only an increase in manufacturing costs of the stainless steel, but also the growth of the oxide film in the operating environment of the fuel cell, thereby having a negative influence upon long term performance. Furthermore, an excessive increase in contents of chromium and nickel deteriorates formability of the metallic bipolar plate, thereby making it difficult to obtain a complex and accurate fluid passage.
Another attempt to enhance electrical conductivity and corrosion resistance of the metallic bipolar plate for fuel cells is disclosed in U.S. Pat. No. 6,440,598 B1. In this disclosure, enhancement in the electrical conductivity of the bipolar plate is attempted by forming a carbon coating layer on a metal plate to prevent surface oxidation of the metal plate while providing good electrical conductivity to the bipolar plate through the metal plate having good electrical conductivity.
In U.S. Pat. No. 6,440,598 B1, however, when used in an environment of severe vibration as in vehicles, the fuel cell suffers powdering of carbon particles which constitute the carbon coating layer, that is, a phenomenon wherein the carbon particles are separated from the coating layer, whereby the separated carbon particles can contaminate the interior of the fuel cell, thereby causing deterioration in overall operation efficiency of the fuel cell.
A further attempt to enhance electrical conductivity and corrosion resistance of the metallic bipolar plate for fuel cells includes plasma coating or physical vapor deposition (PVD) for coating a material exhibiting good electrical conductivity and corrosion resistance onto the surface of the metal plate. However, this process requires a separate space, a so-called chamber, so that a continuous process cannot be used for fabricating the bipolar plate, thereby deteriorating productivity.
Therefore, there is a need for studies into various aspects to develop a method of manufacturing a metallic bipolar plate for fuel cells, which can satisfy the DOE standards in terms of corrosion resistance and contact resistance without any side effect not only initially but also after a predetermined period of time even in an environment of severe vibration as in vehicles while allowing a continuous process with a low manufacturing cost.